The present invention relates to self-powered radiation detectors which are used within the core of a nuclear reactor for neutron flux mapping and to detect sudden changes in local reactivity.
A self-powered radiation detector is one in which an electrical signal is generated between an emitter and collector electrode as a function of the neutron flux without an external drive potential. The signal is produced as a result of the differing radiation response of the different metals which make up the emitter and collector. In general, such devices use a central emitter wire of a metal such as platinum, rhodium, or cobalt. An insulating means is provided about the emitter, and a coaxially disposed collector sheath of a low neutron responsive material, such as nickel-alloy Inconel, is disposed about the insulating means. The detector device is typically less than a quarter inch in diameter and easily is fitted within the fuel assembly of a reactor core.
In a typical nuclear power reactor the neutron spectrum includes a thermal neutron component with a Maxwellian distribution average energy of about KT, which is about 0.04 electron volt at 300.degree. C. Such reactors also include an epithermal neutron component with a flux distribution that falls off as 1/E, and extends from greater than about 0.04 electron volt to about 1 mev. Each of these neutron spectrum components contribute to reactor power, but the thermal component is dominant, contributing about 85-90 percent of the power in a light water reactor of the pressurized or boiling water types. The thermal component contributes over 97 percent of the power in a heavy water reactor such as the Canadian Candu reactor.
It has recently been demonstrated that self-powered detectors respond to both thermal and epithermal neutron flux, in an article "The Epithermal Component in the Neutron Response of Various Self-Powered Detectors" in IEEE Transactions on Nuclear Science, February 1980. In a light water reactor, the signal current generated in a rhodium emitter self-powered detector is attributable equally to thermal and epithermal neutrons. In a cobalt emitter detector about 35 percent of the generated signal is due to epithermal neutrons.
It is also known that the ratio of epithermal to thermal neutron flux varies with position in the reactor core, particularly at the outer edges, as well as being variable with the age of the fuel in the core. In general, the epithermal to thermal flux ratio varies from that at the central core during fuel life by more than 20 percent over about 15-20 percent of the core volume.
The self-powered detector has been used as a neutron flux mapping tool in order to optimize reactor operation from an economic and safety viewpoint. In order to ensure accurate flux mapping the detector response to epithermal flux must also be factored into the flux mapping.
In the above mentioned recent paper, authored by the present inventors, it was shown that the signal current I from a self-powered detector can be expressed as: EQU I=.phi..sub.th S.sub.th +.phi..sub.L S.sub.ep
In the above equation detector current I is in amperes per centimeter of detector active length, .phi..sub.th is the total thermal flux, S.sub.th is the sensitivity of the detector to a thermal neutron flux expressed in amperes per neutron flux per centimeter of detector length, and S.sub.ep is the epithermal neutron sensitivity in the same units, and .phi..sub.L is a measure of the epithermal neutron flux expressed as epithermal neutron flux per unit lethargy. The term lethargy is a term of art referring to the neutron flux per logarithmic energy interval.
The signal current equation above can be used when signal currents are had from the self-powered detectors of the present invention to produce two simultaneous equations in two unknowns, where the unknowns are the thermal and epithermal flux. These equations can then be solved for these fluxes below: ##EQU1##
In U.S. Pat. No. 3,904,881, a self-powered radiation detector is disclosed in which two different emitters are disposed in parallel within a collector sheath. The emitters comprise different materials of differing gamma sensitivity to permit gamma compensation of the output signal. A first emitter is neutron and gamma sensitive and the signal generated therefrom is compared to the separate signal from the other emitter which is gamma sensitive but substantially non-responsive to neutron flux. A variety of gamma compensation techniques are practiced in the prior art, but this is a different function than determining the thermal and epithermal neutron flux values to accurately map core conditions.